The invention relates to methods and apparatus for coating a liquid composition onto a moving support web, and more particularly, to methods and apparatus for increasing the speed of coating application and for improving coating thickness uniformity of applied compositions in instances in which a high degree of uniformity is required.
In the manufacture of many commercial products, such as photographic materials, a liquid composition is applied as a coating to a substrate. In many applications, and especially in imaging films and papers, the requirements for areal uniformity of coated thickness are highly demanding; thickness variations of less than 1% can be unacceptable for some products.
Known curtain coating apparatus typically includes a backing roller around which a web to be coated is wrapped and conveyed at a predetermined conveyance speed. A liquid composition is continuously delivered to and reshaped by an applicator, generally known as a hopper, from a jet flow at the applicator inlet into a broad ribbon of substantially uniform thickness at the applicator outlet from which it is dispensed onto the moving web. Such an applicator is generally positioned above the web at a distance of typically several centimeters, the composition being allowed to fall as a curtain under gravity into continuous contact with the moving web (curtain coating). A liquid composition may be a single layer or a composite layer consisting of a plurality of individual layers of coating compositions.
It is well known that electrostatic field variations created by non-uniform surface charge on the moving web can create objectionable coating non-uniformities. For example, corona discharge treatment of plastic-coated paper to improve adhesion of emulsion to the paper also creates non-uniform charge patterns that cause coating disturbances such as crosslines or mottle. Means for eliminating such charge patterns are disclosed in U.S. Pat. No. 3,531,314 using charged rollers and in U.S. Pat. No. 3,729,648 using ionizers.
The moving web carries with it a boundary layer of air on the front side (the side to be coated) and the back side (the side facing the backing roller). To prevent upsets in the coating and resulting coated thickness nonuniformities, each boundary layer of air must be eliminated before or at the coating point. The elimination of boundary layers becomes more difficult as coating speed is increased.
In all coating systems, there is an upper speed limit for coating at which the boundary layer of air carried on the web surface to be coated is no longer squeezed out by the advancing composition at the coating point, but rather becomes entrained under the composition, disrupting the uniform application thereof to the web and resulting in unacceptable coating non-uniformity.
It is well known that electrostatic charging of a web and/or coating apparatus can be useful in increasing this limit on coating speed, such process being referred to herein as electrostatic assist. For example, a dielectric web carrying a bound polar charge between opposite surfaces thereof can exhibit increased xe2x80x9cwettabilityxe2x80x9d and a consequent increase in acceptable coating speed when conveyed around a grounded coating roller. Means for applying such a charge to a web ahead of the coating point are disclosed, for example, in European Patent No. EP 390774; U.S. Pat. Nos. 4,835,004; 5,122,386; 5,295,039; and European Patent Application No. 0 530 752 A1.
Apparatus and methods also have been proposed for maintaining a uniform charge on a web between the charging apparatus and the coating roller. See, for example, U.S. Pat. No. 4,835,004 and European Patent Application No. 0 530 752 A1 which propose to prevent degradation of charge uniformity by imposing strict environmental controls around the web.
It is also known to apply electrostatic charge at the coating point by electrifying the surface of the coating roller itself. See, for example, U.S. Pat. Nos. 3,335,026; 4,837,045; and 4,864,460.
All of these techniques can be useful in electrostatically assisting the coating of a composition to a web by providing an electrostatic field between the composition and the backing roller at the point of coating. Such an assist acts to cause the composition to be drawn more aggressively toward the backing roller and thus to more forcefully squeeze out the front side boundary layer of air, permitting thereby an increase in coating speed which can be economically beneficial.
As noted above, a moving web also carries a boundary layer of air on its back side or surface as does the backing roller surface prior to engagement with the web. For every conveyance system there exists a speed at which conveyance is limited by back surface air entrainment between the web and the conveying roller. It is known to provide means to remove or exhaust the boundary layers of air being carried on the back surface of a web and the surface of a roller when the two come into contact, increasing thereby the tractional contact of the web with the roller. Such means may include, for example, a pressure-loaded nip roller urged toward the conveying roller, the web passing therebetween. However, use of a nip roller may not be particularly desirable, as it adds mechanical complexity to the apparatus, and a face-side nip roller can mar the surface of the web to be coated and can cause electrostatic disturbance of either or both of the web surfaces, resulting in coating non-uniformities.
Such means may also include a relief pattern formed in the surface of the conveying roller into which the back-side boundary layer air may be exhausted from the web and escape. See U.S. Pat. No. 3,405,855 issued Oct. 15, 1968 to Daly et al., for example. In this patent, Daly et al. teach the use of a roller having peripheral venting grooves and supporting land areas to vent air carried by the underside of the traveling web. Typically, for example, approximately 10% to 40% of the roller surface consists of grooves 0.5 mm to 2.4 mm in depth, 0.5 mm to 2.3 mm in width, and arranged from 5 mm to 15 mm apart. Another example is provided by U.S. Pat. No. 4,426,757 issued Jan. 24, 1984 to Hourticolon, et al. In this patent, Hourticolon, et al. teach the manufacture and use of a roller having a surface relief consisting of a xe2x80x9cfinely branched network of compression chambers,xe2x80x9d allowing the entrained air to be compressed into pockets rather than reducing the web traction. Both of these patents deal with purely conveyance roller issues and neither patent addresses the issue of electrostatic assist with such a roller surface pattern. A pattern on a backing roller creates a variable gap between the surface of the roller and the support being backed; that is, the gap to non-relieved areas of the roller surface is substantially zero, whereas the distance to the bottom of the relief may be up to 2.4 millimeter deep. This variable gap changes the capacitive relationship between the coating fluid and the backing roller, causing non-uniform electrostatic fields at the locus of the coating line. These electrostatic field variations can be comparable in magnitude to the previously described variations arising from non-uniform charge patterns created by corona discharge treatment and resulting in coating disturbances and coated thickness non-uniformities.
U.S. Ser. No. 09/185,045 teaches that the loss in electrostatic force over the relieved portions of the roller is less detrimental than the loss in electrostatic force caused by the intermittent lifting of the web from the backing roller while conveying at high coating speed (xe2x89xa775 meters per minute or 125 centimeters per second) over a backing roll of large diameter (xe2x89xa710 centimeters). Therefore, to prevent air entrainment between the web and the coating fluid, it is advantageous to use a relieved backing roller when using electrostatic assist at high coating speeds with a large backing roller.
For many coatings requiring only a modest level of uniformity, coating thickness variations on the order of 1% RMS (root mean square) or more, invention described in U.S. application Ser. No. 09/185,045 provides an adequate means to increase coating speeds resulting in acceptable coatings. However, it has been shown that the electrostatic force variations due to the relief pattern can cause non-uniformities in the resulting coating (see U.S. Pat. No. 5,609,923). For very sensitive coatings requiring a high degree of uniformity, coating thickness variations on the order of 0.3% RMS or less, the non-uniformities due to the use of a patterned roll with electrostatic assist can be prohibitive.
Thus there is a need for a method for coating a liquid composition to a moving web at high speed to produce a very uniform coated layer, wherein the backing roller is relieved in a pattern over a substantial portion of its cylindrical surface.
It is an object of the invention to provide an improved web coating method whereby a predetermined, uniform electrostatic charge on a relieved coating roller assists in providing a coating having excellent thickness uniformity and wherein the coating is applied to a moving web supported by a relieved backing roller.
It is a further object of the invention to provide an improved web coating method whereby webs may be coated to an excellent level of uniformity at high coating speeds.
It is a still further object of the invention to provide an improved, more operationally robust web coating method that is more tolerant of other operational variability.
Briefly stated, the foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by partially wrapping the moving web around the backing roller, the backing roller including a relief patterned area on the surface thereof, the relief patterned area including relieved features and non-relieved features, the relief patterned area being at least 30% of the width of the moving web, the relief patterned area being circumferential of the backing roller, the relieved features and the non-relieved features creating an electrostatic force difference exerted on the liquid composition at the coating point when an electrostatic field is applied thereto; specifying a predetermined acceptable level of coating thickness non-uniformity, the level of coating thickness non-uniformity increasing with an increase in electrostatic force difference and decreasing with an increase in web speed; specifying a web speed; varying the electrostatic force difference exerted on the liquid composition at the coating point, the electrostatic force difference determined by the relief patterned area, a capacitance of the moving web per unit area, and an electrostatic charge coating assist level, to determine a maximum electrostatic force difference for the specified web speed that achieves the predetermined acceptable level of coating thickness non-uniformity; dispensing the liquid composition from a curtain coating apparatus with a curtain height greater than or equal to 5 centimeters onto the moving web at the coating point; moving the web at the specified web speed; and generating an operating electrostatic force difference at the coating point that is not greater than the maximum electrostatic force difference for the specified web speed.
Alternatively, the curtain coating method of the present invention may be practiced by partially wrapping the moving web around the backing roller, the backing roller including a relief patterned area on the surface thereof, the relief patterned area including relieved features and non-relieved features, the relief patterned area being at least 30% of the width of the moving web, the relief patterned area being circumferential of the backing roller, the relieved features and the non-relieved features creating an electrostatic force difference exerted on the liquid composition at the coating point when an electrostatic field is applied thereto; specifying a predetermined acceptable level of coating thickness non-uniformity, the level of coating thickness non-uniformity increasing with an increase in electrostatic force difference and decreasing with an increase in web speed; specifying an electrostatic force difference exerted on the liquid composition at the coating point, the electrostatic force difference determined by the relief patterned area, a capacitance of the moving web per unit area, and an electrostatic charge coating assist level; varying the web speed to determine a minimum web speed for the specified electrostatic force difference that achieves the predetermined acceptable level of coating thickness non-uniformity; dispensing the liquid composition from a curtain coating apparatus with a curtain height greater than or equal to 5 centimeters onto the moving web at the coating point; generating the specified electrostatic force difference at the coating point; and moving the web at an operating speed that is not less than the minimum web speed for the specified electrostatic force difference.
In the practice of the method of the present invention, the electrostatic field extends through the web to produce an electrostatic xe2x80x9cpressurexe2x80x9d or xe2x80x9cforcexe2x80x9d urging the liquid composition toward the front surface of the substrate at the coating point to exclude the front side air boundary layer, and the relief patterned surface of the backing roller dissipating the back side air boundary layer. It would be expected that using a relief patterned surface on the backing roller in an electrostatically assisted coating process would result in the reproduction of the pattern of the relief patterned surface in the coating laydown as a result of the electrostatic field variations at the liquid-air interface of the coating composition. Surprisingly, practicing the curtain coating method in the above described manner permits high coating speeds with essentially no reproduction of the relief pattern into the final coated thickness of the coating composition, the method being effective with both single- and multi-layer coatings.
In the practice of a preferred embodiment of the method and apparatus of the present invention, a substantially dielectric web to be coated (for example, a web formed from polyethylene terephthalate to be coated either with a single or multiple coatings of a gelatin-based aqueous emulsion) is first passed through means for dissipating all surface charges on the web. Preferably such means is disposed in the web conveyance path of a coating machine a short distance ahead of the point of entrance of the web onto the coating backing roller. An example of a suitable means for dissipating charges is a set of ionizers similar to that disclosed in U.S. Pat. No. 3,730,753 to Kerr, hereby incorporated by reference, wherein the web is exposed sequentially to one or more high positive charges and high negative charges to xe2x80x9cfloodxe2x80x9d pre-existing charge variations on the web and is then discharged. Preferably, the web is also conditioned for coating by removal of residual free charge by treatment, for example, in accordance with the disclosure of U.S. Pat. No. 5,432,454, hereby incorporated by reference, as described in detail hereinbelow.
After being electrically neutralized, the web is entered onto an electrically-isolated backing roller having a relief pattern. The electrostatic stress variation experienced by the coating liquid over a relieved and non-relieved portion of the surface pattern can be characterized using the combination of the web capacitance per unit area and the dimensions of the relief pattern, and is represented by the normalized electrostatic force per unit area difference Fdif. The relief pattern covers the entire extent of the roller surface and has a web-roller contact area not greater than 70% (measured as the percentage of the roller surface that is non-relieved). The backing roller is located within a coating station wherein a coating applicator, for example, a hopper, provides a ribbon of liquid composition at a height greater than or equal to 5 centimeters above the backing roller for coating, according to the curtain coating method disclosed by J. F. Greiller (U.S. Pat. No. 3,632,374) and by D. J. Hughes (U.S. Pat. No. 3,508,947), thereby allowing the coating composition to enter a free-fall between the lip of the applicator and the surface of the web backed by the backing roller.
After determining a maximum coated thickness variation or non-uniformity NU and a preferred coating speed S, the following equation is used to determine the maximum allowable electrostatic charge coating assist level Vassist that can be applied while achieving the coated thickness non-uniformity specification,                     NU        =                              a            1                    +                                    a              2                        ⁢                          V              assist              2                        ⁢                          xe2x80x83                        ⁢                                          F                dif                            S                                                          (        1        )            
wherein a1 and a2 are empirical constants.
The applicator is maintained at ground potential, and the roller is maintained at a DC potential of Vassist, either positive or negative, with respect to ground, creating an electrostatic field around the roller. The electrostatic field produces an electrostatic force that acts to propel the emulsion against the web, squeezing out the boundary layer of air being carried on the front surface of the web. At the same time, the patterned relief on the backing roller surface acts to vent the boundary layer of air being carried on the back surface of the web, increasing thereby the tractional contact of the web with the backing roller.
The practical result is enhanced apparent xe2x80x9cwettabilityxe2x80x9d of the web surface and an increase in the maximum coating speed achievable without onset of air entrainment at the coating point or disengagement of the web from the backing roller surface. Additionally, using the relationship described above in Equation 1 to determine either a minimum coating speed or a maximum electrostatic charge level, coating thickness variations can be kept at or below a pre-determined acceptable level in spite of electrostatic force variations created by the relief pattern, thereby permitting electrostatically-assisted coating over a patterned roller at high speeds for coatings very sensitive to variations in coated thickness